Attention is a critical factor in sensory and cognitive performance. Problems in controlling and directing attention can lead to serious deficits in performance and behavior. More complete information about the neural mechanisms underlying attention will be important for understanding how it facilitates perception and guides behaviors. The proposed experiments will extend our understanding of the neuronal basis of attention by testing the hypothesis that attentional modulation depends on the same neuronal circuits that mediate sensory response normalization. Response normalization is a well-established mechanism that adjusts sensory signals based on the overall activity in a large group of neurons with similar response properties. We propose that attentional modulations in visual cerebral cortex act through this normalization mechanism, such that attentional modulation cannot occur in conditions when normalization is weak or eliminated. We will test this hypothesis by using extracellular recordings to measure directly both attentional modulation and response normalization in the activity of individual neurons in cerebral cortex. One specific aim is to extend preliminary observations on the relationship between spatial attention and response normalization in two ways. First, although preliminary results have shown a correlation between attention and normalization across neurons in the middle temporal visual area (MT), we will examine that relationship within neurons by varying the spatial separation between pairs of stimuli in a way that will modulate normalization. Second, we will examine the relationship between spatial attention and response normalization in area V4, to see whether they are correlated in a visual area with markedly different properties. The second specific aim is to examine whether feature-based attention, like spatial attention, is also correlated with normalization. In one set of experiments, we will examine direction-based attention in MT, because direction-based attention is known to be robust in MT. In other experiments we will examine speed- based attention MT, which will provide a valuable comparison because speed is represented in a different functional architecture than direction in MT. Collectively these experiments will provide a critical test of the hypothesis that attention depends on normalization mechanisms. In doing so they have the potential of greatly extending our understanding of the neural mechanisms that underlie attention. PUBLIC HEALTH RELEVANCE Attention is critical to perceptual and cognitive performance, and attention deficits are the most commonly diagnosed behavioral disorder of childhood, with attention deficit hyperactivity disorder (ADHD) affecting as many as 5% of children in the United States [35]. Better understanding of basic neuronal mechanisms related to attention and their interaction with sensory signals is needed for guiding assessment, diagnosis and treatment of deficits of attention. The proposed research will investigate how attention affects visual signals in cerebral cortex, and in particular how it interacts with sensory mechanisms involved in the integration of multiple visual stimuli.